Using a 555, this configuration appears elsewhere on this site. This is from American documentation from the 90s. The indicator can be an LED or some device controlled by a relay.
Using the 555 this circuit was found in a Popular Electronics from the 90's. The circuit directly...
This circuit was found in the December 1971 issue of Radio Electronics magazine. The circuit is...
This circuit is suggested by Texas Instruments in his Optoelectronics Sourcebook. Any photo...